Conventional golf balls include multi-layer balls, which may have one or more wound layers. The difference in playing characteristics is significant among the three different types of balls: two-piece balls, wound balls and multi-layer balls. Two-piece balls are typically made with a single solid core encased by a cover material. These balls are generally most popular among recreational golfers, because they are durable and provide maximum distance. Typically, the solid core is made of polybutadiene chemically cross-linked with zinc diacrylate and/or similar cross-linking agents. The cover material comprises tough, cut-proof blends of one or more ionomers, such as SURLYN® sold commercially by DuPont or IOTEK® sold commercially by Exxon.
Wound balls typically have either a solid rubber or liquid-filled center, around which many yards of a stretched elastic thread or yarn are wound to form a core. The wound core is then covered with a durable ionomer cover, or a softer cover such as balata, polyurethane or polyurea. Wound balls are generally softer than two-piece balls and can provide more spin, thus enabling skilled golfers to have more control over the ball's flight and placement.
Solid multi-layer golf balls may have one or more core layers, one or more intermediate layers and one or more cover layers. They are designed to overcome some of the undesirable features of conventional two-piece balls, such as hard feel, while maintaining the positive attributes of wound balls, such as increased initial velocity and distance. It is also desirable that multi-layer balls have similar “click and feel” and spin characteristics of wound balls.
Solid multi-layer golf balls can be produced using a variety of manufacturing techniques. For example, two or more cover layers may be molded around a conventional core with one or more intermediate layers interposed between the cover layers and the core. Alternatively, multi-layer balls may be formed from cores having more than one core layers and may optionally contain one or more intermediate and/or cover layers. Multi-layer balls may even comprise a conventional wound core around which at least one intermediate layer and/or at least one cover layer are formed. Typically, the outer layers of multi-layer golf balls are formed by molding them around the core or the preceding intermediate layer or cover layer. Conventional techniques for applying such layers include injection molding, compression molding and casting the layer material around the preceding core or layer.
Intermediate layers have been used as water vapor barrier layer, as disclosed in U.S. Pat. No. 6,632,147, and as thin layer of either high density or low density to alter the rotational moment of inertia of the ball, as disclosed in U.S. Pat. Nos. 6,494,795, 6,685,580 and 6,852,042. Intermediate layers can also be non-continuous or do not completely cover the core, as disclosed in U.S. Pat. No. 6,743,123.
U.S. Pat. No. 6,793,867 teaches a method of manufacturing golf balls that prevents the cover from cracking due to thermal expansion of the core and/or the intermediate layer caused by the exothermic reaction released during the formation of the cover. However, there is no known disclosure that teaches the expansion of the intermediate layer to impart desirable property(ies) to the golf ball.